Lastly, diorite and andesite are intermediate rocks, i.e., their composition lies between felsic and mafic). On the other hand, basalt and gabbro are dark-colored mafic rocks (high in iron and calcium) with plagioclase feldspar and pyroxene with smaller amounts of biotite and hornblende. Granite and rhyolite are light-colored, felsic rocks or silica-rich with main quartz and felspar and smaller amounts of mica, hornblende, etc. If you consider their chemical composition or mineralogy, basalt is the volcanic equivalent of gabbro, granite the equivalent of rhyolite, and andesite the equivalent of diorite. The typical examples of aphanitic rocks are basalt, rhyolite, and andesite, while those of phaneritic texture are granite, gabbro, and diorite. What are the common examples of aphanitic and phaneritic rocks? It only indicates two cooling stages, i.e., slow that forms the larger crystals and final faster cooling forms finer-grained matrix. However, aphanitic and phaneritic rocks can show porphyry or a porphyritic texture. They may have an aphanitic texture but, more often, have a porphyritic texture. Subvolcanic forms at depths less than 1.2 miles (2 km) from the Earth’s surface, especially in sills, dykes, or laccoliths. However, some aphanitic textured rocks are intrusive, i.e., hypabyssal, the subvolcanic intrusive rock. In contrast, aphanitic is mainly extrusive or volcanic igneous rock. The phaneritic texture is an intrusive igneous rock texture since these rocks form deep inside the Earth’s crust. Rock types – Intrusive or extrusive igneous rock? Also, you can know the depths at which cooling occurred. In aphanitic texture, the cooling rate was fast, while phaneritic had a slow speed for phaneritic.

Therefore, you can tell the cooling history by considering these two textures. Phaneritic texture forms from the slow cooling (allows crystals to grow large) of magma deep in the Earth’s crust, while aphanitic texture forms on or near the Earth’s surface where fast cooling doesn’t allow crystals to grow large. For instance, a rock with 90% of the mineral crystals below 1/16mm is aphanitic, even if there are a few larger crystals. Lastly, for an igneous rock to be classified as phaneritic or aphanitic, most grain sizes, i.e., at least 90%, must be within the size range.

Our eyes can see a grain of about 1/10mm.Īlso, note that rock with grain-sizes larger than 3cm is said to have pegmatitic texture and forms in the last stages of magma crystallization.

The emphasis is on the smallest mineral grain you can see with an unaided eye and what you cannot. There is no unanimity on the respective sizes of phaneritic and aphanitic grain. However, Encyclopedia Britannica put the measures for phaneritic and aphanitic textures to 1/10mm to 3cm and below 1/10mm, respectively. The grain size of phaneritic textured rock ranges from 1/16mm to 3cm, while that of aphanitic is below 1/16mm. On the right: Dark colored aphanitic rock whose fine grains are so small that you cannot see them with your naked eyes. The black crystals of hornblende amphibole. You can see quartz (glassy grayish), plagioclase felspar (whitish), potassium feldspar (pinkish). On the Left: Granite, a phaneritic rock with large visible crystals.

Dexture meaning